International Journal of Molecular Sciences Article Bull Sperm Capacitation Is Accompanied by Redox Modifications of Proteins Agnieszka Mostek *, Anna Janta , Anna Majewska and Andrzej Ciereszko Department of Gamete and Embryo Biology, Institute of Animal Reproduction and Food Research of Polish Academy of Sciences, 10-748 Olsztyn, Poland; [email protected] (A.J.); [email protected] (A.M.); [email protected] (A.C.) * Correspondence: [email protected]; Tel.: +48-89-5393134 Abstract: The ability to fertilise an egg is acquired by the mammalian sperm during the complex biochemical process called capacitation. Capacitation is accompanied by the production of reactive oxygen species (ROS), but the mechanism of redox regulation during capacitation has not been elucidated. This study aimed to verify whether capacitation coincides with reversible oxidative post-translational modifications of proteins (oxPTMs). Flow cytometry, fluorescence microscopy and Western blot analyses were used to verify the sperm capacitation process. A fluorescent gel-based redox proteomic approach allowed us to observe changes in the level of reversible oxPTMs manifested by the reduction or oxidation of susceptible cysteines in sperm proteins. Sperm capacitation was accompanied with redox modifications of 48 protein spots corresponding to 22 proteins involved in the production of ROS (SOD, DLD), playing a role in downstream redox signal transfer (GAPDHS and GST) related to the cAMP/PKA pathway (ROPN1L, SPA17), acrosome exocytosis (ACRB, sperm acrosome associated protein 9, IZUMO4), actin polymerisation (CAPZB) and hyperactivation Citation: Mostek, A.; Janta, A.; (TUBB4B, TUB1A). The results demonstrated that sperm capacitation is accompanied by altered Majewska, A.; Ciereszko, A. Bull levels of oxPTMs of a group of redox responsive proteins, filling gaps in our knowledge concerning Sperm Capacitation Is Accompanied sperm capacitation. by Redox Modifications of Proteins. Int. J. Mol. Sci. 2021, 22, 7903. Keywords: sperm capacitation; Bos taurus; redox proteomics; reversible oxPTMs https://doi.org/10.3390/ijms22157903 Academic Editors: Katerina Komrskova, Pavla Postlerova and 1. Introduction Michaela Frolikova Mammalian sperm capacitation is a complex process that occurs in the oviduct and is essential for the fertilisation of a mature oocyte. The successful completion of capacitation, Received: 17 June 2021 which involves many biochemical and morphological changes, enables the spermatozoon Accepted: 19 July 2021 Published: 23 July 2021 to bind to the zona pellucida, penetrate it and fuse with the oolemma. Biochemical changes in the plasma membrane and other subcellular compartments have been associated with Publisher’s Note: MDPI stays neutral sperm capacitation [1]. At the early stages of capacitation, a series of cellular events occurs. with regard to jurisdictional claims in These include, among others, the production of cAMP by the adenylyl cyclase (AC), the published maps and institutional affil- activation of calcium channels, reactive oxygen species (ROS) generation, cholesterol efflux iations. from the plasma membrane, the alkalinisation of sperm plasma and the activation of protein kinases [2]. It is well known that mammalian sperm capacitation is an oxidative process [3,4]. The production of ROS is an early event during the series of modifications that occur then, and a variety of ROS species are produced while spermatozoa capacitate [3,5]. ROS Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. may be produced in different sperm cell compartments; however, it remains unknown This article is an open access article which source exactly is responsible for generating ROS that are involved in capacitation [6]. distributed under the terms and Growing evidence suggests that oxidases localised in the sperm plasma membrane, such conditions of the Creative Commons as nitric oxide synthase (NOS) or oxidase with intrinsic NOS activity, are the main source Attribution (CC BY) license (https:// of ROS and reactive nitrogen species (RNS) in bull and human spermatozoa [3]. ROS and •− creativecommons.org/licenses/by/ RNS are formed by the action of O2 and redox enzymes. Although a small percentage of 4.0/). H2O2, one of the most abundant ROS, may be generated spontaneously, the majority is Int. J. Mol. Sci. 2021, 22, 7903. https://doi.org/10.3390/ijms22157903 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, 7903 2 of 21 •− formed as a result of O2 dismutation catalysed by superoxide dismutases (SODs) and other oxidases with dismutase activity. Most OH•, on the other hand, is produced from •− - H2O2 and O2 in the reaction catalysed by iron or copper ions [7]. In turn, ONOO is •− formed in the reaction between NO and O2 . ROS generation causes the oxidation of cholesterol to oxysterols, which results in a dramatic increase in membrane fluidity. The •− − 2+ combined actions of O2 , HCO3 and Ca activate soluble adenylyl cyclase (sAC), which stimulates cAMP production and the further activation of protein kinase A (PKA). In all species studied to date, including bull, this reaction leads to tyrosine phosphorylation [6]. •− Sperm mitochondria are an important source of O2 , generating low levels of ROS during steady-state respiration. Given that mitochondria are extremely susceptible to •− electron leakage and O2 easily passes through the membrane via voltage-dependent anion channels, mitochondrial ROS can be extremely damaging to the cell [6]. A major, often overlooked, source of ROS is dead or damaged sperm cells with impaired mitochondria [8]. The excessive accumulation of reactive oxygen and nitrogen species may lead to oxidative stress, especially when this is accompanied by insufficient activity of ROS scavenging systems [9]. On the other hand, the precisely localised and controlled synthesis of ROS and RNS in cells can be beneficial in certain physiological states. Various reversible oxidative modifications, known as redox signalling, are triggered by ROS and RNS and are recognised as an important element of the regulatory mechanism of cell metabolism [10,11]. A key role in redox signalling is played by highly reactive protein thiol groups on cysteine residues. They easily react with ROS and RNS and form either reversible or irreversible oxidative post-translational modifications (oxPTMs) which have a substantial impact on protein function [12]. Reversible oxPTMs of cysteines, such as the formation of disulphide bridges, S-nitrosylation, sulphenylation and S-glutathionylation, can be transformed into other stable oxidised groups or reduced back to form free thiols. Cysteine sulphonation can only be reduced back by sulphiredoxin or sestrin. In contrast, the irreversible covalent sulphinylation of cysteine residues in tryptophan, tyrosine, lysine and histidine may result in the loss of protein function due to improper folding, aggregation or degradation and often coincides with cellular damage [12]. It is assessed that around 10% of all the cysteine residues present in the cell proteome are susceptible to reversible redox modifications [13]. The information contained within the redox status of the protein, including the site, type and extent of the oxidative modification, is transduced through the cell to induce certain molecular responses. Redox signalling via reversible oxPTMs leads to alterations in redox-sensitive target proteins by affecting their activity, stability, localisation and interactions with other molecules. As such, the reversible oxPTMs may regulate a plethora of cellular processes under both physiological and pathological conditions [11]. It should be emphasised that the complexity of cell signal transduction is further increased by the cross-talk between redox signalling via reversible oxPTMs and phosphorylation-based signalling systems [14,15]. According to the current knowledge, sperm capacitation is accompanied by an in- creased concentration of ROS and RNS that triggers the redox signalling cascade. However, the mechanism of redox signalling during sperm capacitation remains unknown. Thus far, the involvement of a small group of redox-regulated proteins, including PKC, Ras and AC, has been shown [3]. Our research was established based on two facts: reversible oxPTMs are a hallmark of redox signalling, and redox modifications of proteins can be an integral mechanism of capacitation. Therefore, in our study, we aimed to investigate whether bull sperm capacitation is accompanied by reversible oxPTMs of proteins, which may indicate a redox signal transmission. 2. Results 2.1. Evaluation of Sperm Capacitation: CTC Staining, LPC-Induced Acrosome Reaction and Tyrosine Phosphorylation Level A constant increase in the fraction of capacitated spermatozoa was observed over time in capacitation-supporting buffer as chlortetracycline (CTC)-stained pattern B (Figures1 and2A) . A significant difference was observed after 2 h of incubation, and the largest accumulation Int. J. Mol. Sci. 2021, 22, 7903 3 of 21 of pattern B among different timepoints was observed after 6 h of capacitation, reaching 31 ± 8.1% (Figure2A). The number of sperm showing a spontaneous acrosomal reaction increased with the duration of capacitation. The levels of acrosome reaction induced by lysophosphatidylcholine (LPC), which is a determinant of capacitation, increased significantly after 4 h and further after 6 h of incubation, finally reaching 53 ± 4.4% (Figure2B). The incubation of the bull sperm under conditions supporting capacitation